Methods and apparatus for imaging in scattering environments

ABSTRACT

Systems and method for imaging through scattering media. One example of an imaging system includes an illuminator configured to produce a structured illumination pattern and to direct the structured illumination pattern into an imaging field through a scattering medium, a camera configured to receive reflections of the structured illumination pattern from an object in the imaging field and to provide corresponding image data, a master oscillator coupled to the illuminator and to the camera and configured to modulate the structured illumination pattern and to clock the camera so as to time gate the reflections of the structured illumination pattern and provide signal timing data, and a processor coupled to the camera and received to receive and process the image data and the signal timing data to produce an image of the object.

BACKGROUND

Optical imaging through turbid (e.g., cloudy) water results in poorsignal-to-noise ratio data, compromising qualitative and quantitativeanalyses or requiring unacceptable operational constraints. Currentsolutions are technically complex and expensive, limiting their abilityto be used in emerging low cost architectures. For example, certainconventional systems use Light Distancing and Ranging (LIDAR) whichexcludes out-of-object-plane light using time gating. However, suchsystems are generally large and expensive, and do not providecolor-corrected images. Other approaches involve placing the opticalsensor close to the object being imaged, which reduces the impact ofscattering exponentially. However, for many applications requiringclose-range imaging is undesirable and potentially risky. Someapproaches focus on image processing to “clean up” images withscattering effects, but have limited ability to improve image quality.Photonic mixing devices (PMDs) also enable time of flight measurementsfor range finding applications. A light source is modulated at afrequency in the range of 1 MHz to 1 GHz. The light illuminates a scene,and part of the reflected light enters the range finder camera. Bymeasuring in each pixel the phase of the incident light, a distance canbe estimated between the pixel and its conjugate (light-reflecting)pixel area in the scene. In this way the distances of objects and shapeof objects can be estimated and recorded. As opposed to some externallymixed schemes, a PMD mixes the incident light immediately in thedetector, instead of in a connected electronic mixer. As a result, lownoise may be achieved, and often a better signal to noise ratio and asmaller error on the estimated distance.

SUMMARY OF INVENTION

As discussed above, conventional technologies for imaging throughscattering media in either air or water are expensive and/or complex andhave limited success. Additionally, conventional systems do not providecolor images of the object. Accordingly, a need exists for a relativelysmall, inexpensive imaging system capable of providing high qualitycolor images in turbid marine environments in a stand-off geometry.

According to one embodiment, an imaging system comprises an illuminatorconfigured to produce a structured illumination pattern and to directthe structured illumination pattern into an imaging field, a cameraconfigured to receive reflections of the structured illumination patternfrom an object in the imaging field and to provide corresponding imagedata, a master oscillator coupled to the illuminator and to the cameraand configured to modulate the structured illumination pattern and toclock the camera so as to time gate the reflections of the structuredillumination pattern and provide signal timing data, and a processorcoupled to the camera and received to receive and process the image dataand the signal timing data to produce an image of the object.

In one example, the camera includes an array of imaging pixels, eachpixel including a photosensitive detector configured to receive thereflections of the structured illumination pattern and to produce theimage data, and a phase meter coupled to the master oscillator. In oneexample, the illuminator is configured to produce the structuredillumination pattern having a wavelength in the green region of thevisible spectrum. In another example, the structured illuminationpattern includes an array of dots. The processor may be furtherconfigured to determine a scattering coefficient of a scattering mediumthrough which the structured illumination pattern and the reflections ofthe structured illumination pattern travel based on analysis of theimage data. In one example, the system further includes at least one RGBcamera coupled to the processor and configured to produce an RGB imageof the object. The processor may be further configured to analyze theimage data to obtain an estimated range between the object and thecamera, and to overlay the estimated range with the RGB image to producethe image of the object. In one example, the system further includes anarrowband filter coupled to the at least one RGB camera and configuredto exclude the reflections of the structured illumination pattern frombeing received by the at least one RGB camera.

According to another embodiment, a method of imaging an object in ascattering medium comprises acts of generating a time-modulatedstructured illumination pattern, illuminating the object with thetime-modulated structured illumination pattern, receiving a reflectedsignal at an imaging sensor, the reflected signal including reflectionsof the structured illumination pattern from the object and backscatterfrom the scattering medium, collecting spatially resolved range-gatedimage data by time gating the reflected signal to exclude thebackscatter, and processing the image data to produce an image of theobject.

In one example, the act of generating the time-modulated structuredillumination pattern includes modulating an illumination source using amaster oscillator, and the act of time gating the reflected signalincludes clocking the imaging sensor using the master oscillator. Inanother example, the method further includes producing a passiveillumination RGB image of the object. In one example, the act ofprocessing the image data includes extracting spatial backscattersignatures from the image data, and determining a scattering coefficientof the scattering medium. In one example, the act of extracting thespatial backscatter signatures from the image data includes measuring asignal strength of the backscatter. The method may further includecorrecting the RGB image of the object using the scattering coefficientto produce a corrected image of the object. In one example, the act ofprocessing the image data includes determining an estimated range to theobject and a shape of the object; and further comprising overlaying theestimated range with the RGB image to produce the image of the object.In another example, the act of producing the RGB image is performedusing an RGB camera, and further comprising filtering the RGB camera toexclude the reflections of the structured illumination pattern. Inanother example, the act of generating the time-modulated structuredillumination pattern includes generating the time-modulated structuredillumination pattern having a wavelength in the green region of thevisible spectrum.

According to another embodiment, an imaging system comprises anilluminator configured to produce an illumination beam having awavelength in the green region of the visible spectrum and to direct theillumination beam into an imaging field, a camera configured to receivereflections of the illumination beam from an object in the imaging fieldand to provide corresponding image data, a master oscillator coupled tothe illuminator and to the camera and configured to modulate theillumination beam and to clock the camera so as to time gate thereflections of the illumination beam and provide signal timing data, anda processor coupled to the camera and received to receive and processthe image data and the signal timing data to produce an image of theobject.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments are discussed in detail below. Embodimentsdisclosed herein may be combined with other embodiments in any mannerconsistent with at least one of the principles disclosed herein, andreferences to “an embodiment,” “some embodiments,” “an alternateembodiment,” “various embodiments,” “one embodiment” or the like are notnecessarily mutually exclusive and are intended to indicate that aparticular feature, structure, or characteristic described may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of at least one embodiment are discussed below withreference to the accompanying figures, which are not intended to bedrawn to scale. The figures are included to provide illustration and afurther understanding of the various aspects and embodiments, and areincorporated in and constitute a part of this specification, but are notintended as a definition of the limits of the invention. In the figures,each identical or nearly identical component that is illustrated invarious figures is represented by a like numeral. For purposes ofclarity, not every component may be labeled in every figure. In thefigures:

FIG. 1 is a functional block diagram of one example of an imaging systemconfigured to implement time-of-flight processing according to aspectsof the invention;

FIG. 2 is a functional block diagram of one example of an imaging systemconfigured to implement structured illumination according to aspects ofthe invention; and

FIG. 3 is a functional block diagram of another example of an imagingsystem according to aspects of the invention.

DETAILED DESCRIPTION

Near-object underwater inspection is an area of increasing interest forseveral applications, including in mine clearance with unmannedunderwater vehicles (UUV). These and other underwater imaging systemstypically include a single visible-spectrum optical sensor. As discussedabove, one challenge to the successful use of these optical sensorsresults from the turbidity or cloudiness of the water. General sourcesof turbidity generally include small particulates, microscopic marinefauna and flora and other suspended particles in the water column. Lightthat is scattered from the entire volume of water (referred to herein as“out-of-plane scatter”) creates poor signal-to-noise ratio (S/N) opticalimages, or requires that the sensor be proximal to the object beinginspected. These are challenges to both qualitative and quantitativeanalysis and characterization of underwater objects.

Accordingly, aspects and embodiments are directed to systems and methodsfor imaging objects through a moderately scattering medium, such asturbid water, for example. In particular, certain embodiments aredirected to short to mid-range optical imaging in turbid marineenvironments, including the use of modified commercial off-the-shelf(MOTS) sensors to exclude out-of-plane optical scatter. As discussed inmore detail below, time-of-flight (TOF) processing may be used toexclude light that is scattered out of the object plane and wouldotherwise degrade the image. This processing can be principally used toreduce or remove backscattered signal from objects. Additionally, theTOF processing may be combined with polarization or other methods toreduce the lesser effects of forward scattering image noise. Thisapproach includes the use of a TOF camera (imaging detector or sensor)that may be modified for use in marine environments. In addition, astructured illumination pattern may be projected onto the target object,and image processing may be used to extract image boundaries, distanceand other information by monitoring changes to that pattern. A standardimage overlay may be made, and additional image processing can be usedto provide a final image. This approach may also include the use of aMOTS sensor, as discussed further below. Certain embodiments mayimplement TOF processing alone, or structured illumination (SI) alone,while other embodiments may implement a combination of the twotechniques, as discussed further below. Additionally, embodiments of theimaging system may be configured to extract scattering informationdirectly from the scattering medium, allowing for further imageprocessing, as also discussed below. The underlying imaging sub-systemsused in each approach may include red-green-blue (RGB) cameras, aranging sensor, an illumination source, and associated electronics. Suchsub-systems are available at low cost due to high-volume manufacture forexisting commercial applications. Accordingly, aspects and embodimentsimplementing these approaches may leverage high-volume and low cost COTSitems (e.g., sensors and illuminators) to provide an affordable solutionfor quality underwater imaging.

It is to be appreciated that embodiments of the methods and apparatusesdiscussed herein are not limited in application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the accompanying drawings. Themethods and apparatuses are capable of implementation in otherembodiments and of being practiced or of being carried out in variousways. Examples of specific implementations are provided herein forillustrative purposes only and are not intended to be limiting. Also,the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use herein of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof is meant to encompass the items listed thereafter andequivalents thereof as well as additional items. References to “or” maybe construed as inclusive so that any terms described using “or” mayindicate any of a single, more than one, and all of the described terms.

Referring to FIG. 1, there is illustrated a functional block diagram ofone example of an imaging system based on TOF processing, according tocertain aspects. The system includes an illuminator 105 that generatesan illumination beam 110 which may be directed towards an object 115.The system further includes a camera 120 that receives reflected photons125 from the object 115. The illumination beam 110 and reflected beam125 traverse a scattering medium 130, such as turbid water, for example,between the system and the object 115. As discussed above, in a TOFsystem, photons received at the imaging sensor (camera 120) are timegated, which provides ability to slice the resulting image data toaccess the z-direction information (distance from the camera 120), andthereby exclude contributions from scattered reflections which falloutside of the selected time gate. As discussed further below, timegating may be achieved by modulating the illumination beam 110 using amaster oscillator 135 coupled to the illuminator 105. In one example,the master oscillator 135 is an RF source, producing a master signalhaving a frequency of about 20 MHz, for example. The camera 120 isclocked to the master oscillator 135, as shown in FIG. 1, such thattiming information about the signal 125 is collected. In one example,the camera 120 is a solid state camera. In one embodiment, the camera120 includes an array of pixels 140. Each pixel 140 may include aphoto-diode (or other photo-sensitive detector) 145, along with an RFphase meter 150 that is used for timing information, as discussedfurther below.

The illuminator 105 may include an LED or laser diode, for example, thatproduces a beam having a desired wavelength. Illumination sourcescommonly used in other applications (e.g., laser range-finding)typically produce beams having a wavelength in the infrared spectralregion (e.g., approximately 900+ nm). However, optical absorption inwater is almost complete at infrared wavelengths, even over shortdistances, making it virtually impossible to perform underwaterstand-off imaging at these wavelengths. According to certainembodiments, the illuminator 105 is modified, relative to standardconventional illumination sources, to produce the illumination beam 110having a wavelength suitable for underwater imaging, for example, awavelength in the green region of the visible spectrum (e.g., in therange of approximately 520-570 nm). According to Beer's law, a 2 meter(m) total path length through water, corresponding to a separation of 1m between the illuminator 105 and the object 115, results inapproximately a 20% reduction in optical power due to absorption for thegreen region of the spectrum. In contrast, for the near-infrared (NIR)spectral region, optical absorption is approximately 90% over the samepath length.

Thus, using modulated active illumination having a wavelengthappropriately selected for the scattering medium, a modified TOF systemprovides the ability to slice the resulting image data to access thez-direction (distance from the camera 120), and enables the exclusion ofphotons that are scattered out of the object plane and would otherwisedegrade the image. Back-end signal processing may be used to derive thecorrect slice of the image (similar to multipath rejection processing,for example), as will be appreciated by those skilled in the art, giventhe benefit of this disclosure. The signal processing may be performedby a processor/controller 155 coupled to or integrated with the camera120 and that receives the image data from the camera 120. In someexamples, the time gating may be applied directly using knowledge of therange to the object 115, as discussed further below. Provided thatsufficient signal 125 is received by the camera 120, which should be thecase for a representative marine environment, the TOF technique may beused to provide an image substantially free from the effects ofscattering. The image may be displayed on a display 160. In the exampleillustrated in FIG. 1, the display 160 is shown integrated with thecontroller 155; however, in other examples the display 160 may beseparate from and coupled to the controller.

In some embodiments, the imaging system may be further configured toimplement structured illumination, in which the illumination beam isreplaced with a structured illumination pattern, such as an array ofbright dots, for example. Structured illumination may be used alone toprovide information about the object 115, or in combination with TOFprocessing to provide enhanced imaging of the object. FIG. 2 illustratesa functional block diagram of one example of an imaging systemconfigured to implement structured illumination. In such a system, anilluminator 210 is configured to project an illumination pattern 215into the imaging field and onto the object 115. As in the case of theTOF system discussed above, in certain embodiments, the illuminator 210may be modified to emit the illumination pattern at a wavelengthsuitable for underwater imaging, for example, in the blue or greenregion of the visible spectrum. Image processing is used to extractimage boundaries, distance and other information by monitoring changesto that pattern in light 220 reflected from the object 115. For example,a camera 225 may image the size, distortion and location of the brightspots, and determine the range to the object 115 using back-endprocessing, as discussed further below. In one example, in whichstructured illumination is used to provide range information, the camera225 may be configured to measure the distance the light 220 travels toeach pixel within the imaging sensor of the camera, and therebydetermine the range to the object 115. Embedded imaging software mayused to process the image data obtained from the camera 225 and producean image 230 of the object 115, which may be displayed on a display 240.

According to certain embodiments, aspects of TOF and structuredillumination may be combined to provide an imaging system with improvedoverall performance for short to mid-range optical imaging through amoderately scattering medium, such as turbid water, for example. Forexample, a combined system may include an illuminator configured toproject a modulated structured illumination pattern in a suitablewavelength range (for example, in the blue and/or green region of thevisible spectrum), and a TOF camera with associated image processingsoftware adapted to time gate the reflected illumination pattern. TheTOF function rejects out of plane scatter, providing a higher qualityimage, while the structured illumination function may provide finedistance resolution. Furthermore, the system may also include one ormore RGB cameras used to provide additional imaging capabilities,including the option of providing a color corrected image of the object115, as discussed further below.

Referring to FIG. 3, there is illustrated a functional block diagram ofone example of an imaging method 300 according to certain embodiments.Step 310 includes illuminating an imaging field (including the object115), which may be accomplished using an illuminator 105/210. In oneexample, the illuminator is configured to produce a time modulatedillumination beam (step 312). For example, as discussed above withreference to FIG. 1, in one embodiment, the illuminator 105 is coupledto a master oscillator 135 that produces a clock signal (step 360) thatis used to modulate the illumination beam such that the illuminatorproduces a time-modulated illumination beam (step 312). In one example,the illuminator is a monochrome laser illuminator, and is configured toproduce the modulated illumination beam having a wavelength in the blueand/or green region of the visible spectrum. In certain examples, themodulated illumination beam may be further configured to have astructured illumination pattern, as discussed above with reference toFIG. 2.

In step 314, the modulated illumination beam is projected into theimaging field to illuminate the object 115. Returned illumination, whichmay include reflections of the illumination beam from the object 115 aswell as backscatter from the scattering medium 130 (indicated by block320) is collected by an imaging sensor/camera (step 340). In oneembodiment, detecting the returned illumination (step 340) includescollecting spatially resolved range gated image data (step 345).Referring again to FIG. 1, in one embodiment, the RF phase meter 150 ineach pixel 140 of the camera 120 is clocked by the master oscillator 135and used to extract phase data from the received signal 125 which may beused to time gate the signal. In certain embodiments, the clockingparameters for image acquisition (with appropriate time gating) may bederived from sonar or another sensor input that provides initial rangeinformation (step 305). In certain examples, the camera 120 may includea photonic mixing device, and the RF phase meter 150 is used to measure,in each pixel, the phase of the incident light, such that a distance canbe estimated between the pixel and its conjugate (light-reflecting)pixel area in the scene, as discussed above.

After the image data has been collected, the data set is processed (step350) using image processors and software that may be embedded with thecamera 120/225 or included in a separate controller coupled to thecamera(s). The processed data set may be used to produce an image of theobject 115 that is displayed on a display 155/240 or provided to anotherprocessor (not shown) for further analysis and/or processing (step 370).As discussed above, for the TOF function, the processing step 350 mayinclude using range and TOF information to exclude signal contributionsfrom scattered reflections which fall outside of the selected time gateand would otherwise degrade the image quality. Thus, an image may beproduced (at step 370) that is substantially free of the effects ofscattering.

In certain examples, the imaging system further includes one or more RGBcameras that may be used to supplement the TOF and/or structuredillumination imaging functions. In one example, the one or more RGBcameras may passively image the imaging field (i.e., receiving lightfrom the imaging field, separate from the active illumination performedfor the TOF and/or structured illumination imaging functions), includingthe object 115, to obtain a monochrome or color image of the object(step 330). The imaging system may include a multicolor camera, or mayinclude a set of single-color cameras, the outputs of which may becombined to form an RGB image. In another example, a single camera maybe configured to sequentially obtain single-color images, using a serialRGB illumination scheme, for example, and combine those images toproduce a color image of the object. Image data from the RGB camera(s)may be processed in the data processing step 350, as shown in FIG. 3. Insome examples, the RGB camera(s) may be used to enhance the resolutionof the images obtained using the TOF camera. For example, the imageframes of the RGB camera(s) may be synchronized with the TOF camera,such that the images from each are captured simultaneously and may becombined using conventional image stacking/stitching processes.

In conventional combined imaging and ranging systems, an infrared (IR)active illumination source and complementary receiver are used. Thus,the IR ranging function does not disrupt or interfere with the visible(e.g., RGB) imaging because the returned IR ranging beam is out of theimaging band of the visible receivers. However, as discussed above,according to certain embodiments, a visible, specifically blue or green,active illumination beam may be used. Without other measures taken, thereturn beam from the illuminator may overwhelm the RGB camera(s).Accordingly, in certain example, a narrow bandwidth coherent source,such as a DPSS 532 nm laser source, may be used for the illuminator 105,and a matched band-reject filter may be used to block the returnedillumination beam from the RGB camera(s). The narrow band should notnoticeably distort the image color balance. If any color distortion doesarise, the green (or blue) channel of the image may be compensatedeasily, as will be appreciated by those skilled in the art.

According to some embodiments, the data processing step 350 includesprocesses for directly measuring the strength of the backscatter signal(320) received from the scattering medium 130. The received signalstrength at the camera 120 may provide a measure of both the scatteringin the scattering medium and image contrast in the resulting image ofthe object 115. Thus, unlike conventional TOF systems, where the signalstrength is used to improve range information, or as image contrast indirect imaging applications, according to certain aspects, signalstrength is collected as a combined measure of both image contrast andscattering strength in the scattering medium 130. This measurement ofthe strength of the backscatter signal may be used to extract thescattering coefficient of the medium, which may then be used to makecorrections to the image of the object 115 and thereby improve the imagequality. Thus, the data processing step 350 may include a step 353 ofextracting spatial backscatter signatures from a point cloud returnsignal obtained from the camera (in step 340), and determining thescattering coefficient of the scattering medium 130 (step 352). Once thescattering coefficient(s) have been obtained, they can be used inconjunction with the RGB image data received from the one or more RGBcameras (step 330). For example, the scattering coefficient(s) may beused to correct the RGB image(s) for regional scatter (step 351),thereby providing higher quality, true color images of the object 115.Generally, optical scattering is a function of the wavelength of lightbeing scattered. This is often estimated over the visual/near-infraredwavelengths as a linear or exponential dependency and is typically knownfor a medium or can be estimated. This difference in scattering can havean effect similar to the color distortion noted from absorption effects.By explicitly measuring the scattering at a reference wavelength, theeffect of color distortion from scattering can be estimated. Thisscattering spectrum may then provide explicit color correction for anobject at a known range.

In certain examples, the RGB camera(s) may be used in combination withthe structured active illumination discussed above to take asimultaneous visible image of the object 115, which may be overlaid withthe range information. In the data processing step 350, the overlaidvisible image and range information may be processed using a calibrateddiffused illumination “map” (e.g., obtained from the RGB camera) tolocate, identify, and produce the image 230 of the object 115. Forexample, referring to FIG. 3, in using the structured illuminationpattern, the processing step 350 may include analyzing the returnedstructured pattern, and determining range to the object 115 based ondistortion and other changes in the reflected illumination patternrelative to the original projected pattern. As discussed above, thestructured illumination pattern may include an array of bright dots.Accordingly, the data processing step 350 may include a step 354 ofextracting point cloud features from the analyzing the returned array ofdots. The range to the object 115 and shape of the object may bedetermined from analyzing the point cloud features (step 355), asdiscussed above. The RGB image may then be overlaid with the range andshape information (step 356) to provide a final image that can bedisplayed or presented to a user or other system for further analysis(step 370). Scattering information may also be extracted by analyzingthe returned structured illumination, together with the time of flightinformation. In one example, extraction of scattering informationincludes analysis of the convolution of the spatial signal (345) and theTOF camera longitudinal response (steps 351-353). By combining this TOFfunctionality with the spatial illumination functionality, a moreprecise depth measurement may be enabled even when the scattering wouldtypically prevent the use of structured illumination.

Thus, according to certain aspects and embodiments, systems and methodsfor enhanced underwater imaging, particularly in turbid conditions, maybe provided using relatively inexpensive components. The systems andmethods may similarly be used in other scattering environments. Using acombination of the two imaging modalities discussed above, wherein theTOF camera removes the majority of the backscattering effects andstructured illumination is used to provide fine detail, may provide highquality object imaging and distance profiling, not available throughconventional systems. TOF source and sensor gating may eliminate thebackscatter contribution to the signal, and the finer detail of theobject may be profiled using the structured light capability, asdiscussed above. Combined embodiments may thus provide stand-off rangeimaging in moderate turbidity, while also providing fine distanceresolution (e.g., less than 1 cm). Embodiments of these methods usedwith the LIDAR system may provide the capability to obtain a colorunderwater imaging LIDAR system, optionally with a single imaging sensor(camera 120), and may use three-dimensional (3D) LIDAR range parametersthat are set from sonar or other external sensor data (step 305). Asdiscussed above, explicit extraction of the scattering coefficient bywavelength may allow for color correction of objects imaging through thescattering medium 130.

According to another embodiment, higher resolution images may beobtained by multiplexing multiple TOF cameras together. The TOF camerasmay be clocked using a common (shared) timing signal from the masteroscillator, for example, such that the timing information obtained byeach camera is synchronized. The image data from each camera may becombined using image stitching processing, as will be appreciated bythose skilled in the art. As discussed above, in some embodiments, colorimaging may be obtained by combining image data from one or more RGBcameras with the image data from the TOF camera and/or structuredillumination receiver. In another embodiment, simultaneous RGB TOFimaging may be obtained through multiplexing three TOF cameras, eachappropriately bandpass filtered to receive only a single color, andusing a broadband or RGB illumination source 105.

Having described above several aspects of at least one embodiment, it isto be appreciated various alterations, modifications, and improvementswill readily occur to those skilled in the art. Such alterations,modifications, and improvements are intended to be part of thisdisclosure and are intended to be within the scope of the invention.Accordingly, the foregoing description and drawings are by way ofexample only, and the scope of the invention should be determined fromproper construction of the appended claims, and their equivalents.

What is claimed is:
 1. An imaging system for marine environmentscomprising: an illuminator configured to produce a structuredillumination pattern having a wavelength within the green or blue regionof the visible spectrum and to direct the structured illuminationpattern into an imaging field; a camera configured to receivereflections of the structured illumination pattern from an object in theimaging field, and backscatter from turbid water through which thestructured illumination pattern and the reflections of the structuredillumination pattern travel, and to provide corresponding image data; amaster oscillator coupled to the illuminator and to the camera andconfigured to modulate the structured illumination pattern and to clockthe camera so as to time gate the reflections of the structuredillumination pattern and provide signal timing data; at least one RGBcamera configured to image the object and produce an RGB image of theobject; and a processor coupled to the camera and the RGB camera andconfigured to receive and process the image data and the signal timingdata to produce an image of the object, the processor being configuredto: measure a signal strength of the backscatter from the turbid waterat the wavelength within the green or blue region of the visiblespectrum, wherein the signal strength is a combined measure of both ascattering strength of the turbid water and an image contrast, extract ascattering coefficient of the turbid water based on analysis of thescattering strength, and estimate an effect of color distortion based atleast in part on the scattering coefficient of the turbid water at thewavelength within the green or blue region of the visible spectrum, andcolor correct the RGB image based on the estimated effect of the colordistortion.
 2. The imaging system of claim 1, wherein the cameraincludes an array of imaging pixels, each pixel including aphotosensitive detector configured to receive the reflections of thestructured illumination pattern, and the backscatter from the turbidwater, and to produce the image data, and a phase meter coupled to themaster oscillator.
 3. The imaging system of claim 1, wherein thewavelength within the green region or the blue region of the visiblespectrum is a wavelength within the green region of the visiblespectrum.
 4. The imaging system of claim 1, wherein the structuredillumination pattern includes an array of dots.
 5. The imaging system ofclaim 1, wherein the processor is further configured to analyze theimage data to obtain an estimated range between the object and thecamera, and to overlay the estimated range with the RGB image to producethe image of the object.
 6. The imaging system of claim 1, furthercomprising a narrowband filter coupled to the at least one RGB cameraand configured to exclude the reflections of the structured illuminationpattern from being received by the at least one RGB camera.
 7. Theimaging system of claim 1, wherein in imaging the object and producingthe RGB image, the at least one RGB camera is configured passively imagethe object separate from the reflections of the structured illuminationpattern received at the camera.
 8. The imaging system of claim 7,wherein the at least one RGB camera is configured to image the objectsimultaneously with the directed structured illumination pattern.
 9. Amethod of imaging an object in turbid water, the method comprising:generating a time-modulated structured illumination pattern having awavelength within the green or blue region of the visible spectrum;illuminating the object with the time-modulated structured illuminationpattern; receiving a reflected signal at an imaging sensor, thereflected signal including reflections of the structured illuminationpattern from the object and backscatter from the turbid water;collecting spatially resolved range-gated image data by time gating thereflected signal to exclude the backscatter; producing a passiveillumination RGB image of the object using at least one RGB camera;processing the image data to produce an image of the object includingextracting spatial backscatter signatures from the image data anddetermining a scattering coefficient of the turbid water, whereinextracting the spatial backscatter signatures from the image dataincludes measuring a signal strength of the backscatter from the turbidwater at the wavelength within the green or blue region of the visiblespectrum, the signal strength being a combined measure of both ascattering strength of the turbid water and an image contrast; andestimating an effect of color distortion based at least in part on thescattering coefficient of the turbid water at the wavelength within thegreen or blue region of the visible spectrum, and color correcting theRGB image based on the estimated effect of the color distortion.
 10. Themethod of claim 9, wherein generating the time-modulated structuredillumination pattern includes modulating an illumination source using amaster oscillator; and wherein time gating the reflected signal includesclocking the imaging sensor using the master oscillator.
 11. The methodof claim 9, wherein processing the image data includes determining anestimated range to the object and a shape of the object; and furthercomprising overlaying the estimated range with the RGB image to producethe image of the object.
 12. The method of claim 9, further comprisingfiltering the RGB camera to exclude the reflections of the structuredillumination pattern.
 13. The method of claim 9, wherein generating thetime-modulated structured illumination pattern having a wavelengthwithin the green or blue region of the visible spectrum includesgenerating the time-modulated structured illumination pattern having awavelength in the green region of the visible spectrum.
 14. The imagingmethod of claim 9, wherein producing the passive illumination RGB imageof the object includes imaging the object separate from the reflectionsof the structured illumination pattern.
 15. The imaging method of claim14, wherein producing the passive illumination RGB image of the objectincludes imaging the object, with the RGB camera, simultaneously withilluminating the object with the structured illumination pattern.